I was thinking how this could be accomplished with existing MC features, or with AHK.
With AHK one could create a listView (drop-down) at the mouse cursor, which could have MC commands in it - though I'm unsure if it's possible to send MC commands externally.
The other idea of how to do "drop down menus" was, IF beyond the existing (default) "Button Panel/Window", one could create Multiple Button Panels/Windows.
Each Panel would need it's own definable dimensions Row/Columns.
A dialog would be needed to manage the Button Panels (similiar to others MC uses).
Hotkeys (shortcuts) would need to be able to activate (show/hide) a given panel.
Perhaps a MC.ShowButtonPanel "Name|ID" command.
Given those things, one could assign a MC command to the users QuickLaunch to display a given Button Panel (which appears to recall it's last size and position).
Perhaps a checkbox on the panel itself, or within it's options to "Hide after use".
Probably more work than being able to display a Custom User Menu via a MC command though.